1. Field of the Invention
The present invention relates to a mutual induction circuit, and more particularly to a mutual induction circuit which is formed in first and second wiring layers parallel to each other in a vertical direction and is operated based on an input differential signal.
2. Description of the Background Art
In recent years, through the spread of mobile communication terminal apparatuses, typified by a mobile telephone, a variety of types of radio circuits have tended to be incorporated into an integrated circuit. In such a trend, a transformer element, which is an example of a mutual induction circuit highly used in radio circuits, also has tended to be incorporated into the integrated circuit. Three conventional transformer elements will be described below.
FIG. 32A is a top view schematically illustrating a structure of a transformer element as a first exemplary conventional mutual induction circuit (hereinafter, this transformer element is referred to as a “first mutual induction circuit 100” in this “Description of the Background Art” section). FIG. 32B is a schematic view illustrating a cross section of the first mutual induction circuit 100 taken along line V—V shown in FIG. 32A and viewed from the direction of arrow W1. In FIGS. 32A and 32B, the first mutual induction circuit 100 includes a primary coil 101 and a secondary coil 102. Both of the primary and secondary coils 101 and 102 are formed within an insulating layer 103 such that the primary coil 101 is situated immediately below the secondary coil 102. The primary coil 101 is roughly spiral shaped, and has a first input terminal A1 at one end and a second input terminal A2 at the other end. More specifically, the primary coil 101 is shaped as if a circle extends along one plane outwardly from the first input terminal A1 situated at an approximate center of the spiral. The second input terminal A2 is situated at the end of the outer circumferential side of the primary coil 101.
The secondary coil 102 has substantially the same shape as that of the primary coil 101, and is situated at a location to which the primary coil 101 is translated by a predetermined distance along a vertical direction. The secondary coil 102 has a first output terminal A3 at the end of the spiral center side and a second output terminal A4 at the end of the outer circumferential side.
In the above first mutual induction circuit 100, by applying an electrical signal to the first and second input terminals A1 and A2, an electrical signal in accordance with the ratio of the numbers of turns in the primary and secondary coils 101 and 102 is obtained from each of the first and second output terminals A3 and A4.
FIG. 33 is a vertical cross-sectional view schematically illustrating a structure of a transformer element as a second exemplary conventional mutual induction circuit (hereinafter, this transformer element is referred to as a “second mutual induction circuit 200” in this “Description of the Background Art” section). In FIG. 33, the second mutual induction circuit 200 includes a lower chip 201 and an upper chip 202. The lower chip 201 includes a secondary coil 205 formed on an insulating film 204 laminated on a semiconductor substrate 203. Similarly, the upper chip 202 includes a primary coil 208 formed on an insulating film 207 laminated on a semiconductor substrate 206. The lower and upper chips 201 and 202 are bonded together via a polyimide film 209. In this case, the primary and secondary coils 208 and 205 are situated symmetrical to each other with respect to a reference plane RP virtually formed within the polyimide film 209.
In the above second mutual induction circuit 200, by applying an electrical signal to one of the coils 205 and 208, an electrical signal in accordance with the ratio of the numbers of turns in the coils 205 and 208 is obtained from the other of the coils 205 and 208.
FIG. 34A is a top view schematically illustrating a structure of a transformer element as a third exemplary conventional mutual induction circuit (hereinafter, this transformer element is referred to as a “third mutual induction circuit 300” in this “Description of the Background Art” section). FIG. 34B is a cross-sectional view of the third mutual induction circuit 300 taken along line P—P shown in FIG. 34A and viewed from the direction of arrow Q. In FIGS. 34A and 34B, the third mutual induction circuit 300 is formed on a semiconductor substrate 301, and includes a first planar spiral coil 302, a second planar spiral coil 303, and a third planar spiral coil 304. The second planar spiral coil 303 is formed above the first planar spiral coil 302 via a first insulating film 305. In other words, the second planar spiral coil 303 is situated on the first insulating film 305 formed on the first planar spiral coil 302. Similarly, the third planar spiral coil 304 is formed above the second planar spiral coil 303 via a second insulating film 306. The end of the spiral center side of the first planar spiral coil 302 is electrically connected to the end of the spiral center side of the second planar spiral coil 303. Similarly, the end of the spiral outer circumferential side of the second planar spiral coil 303 is electrically connected to a neighborhood of the end of the spiral outer circumferential side of the third planar spiral coil 304.
A first input terminal 307 is formed by a signal line drawn out from a connection between the first and second planar spiral coils 302 and 303. Similarly, a second input terminal 308 is formed by a signal line drawn out from the end of the spiral center of the third planar spiral coil 304. Further, a first output terminal 309 is formed by an end portion on the spiral outer circumferential side of the first planar spiral coil 302, and a second output terminal 310 is formed by an end portion on the spiral outer circumferential side of the second planar spiral coil 304.
In the above third mutual induction circuit 300, by applying an electrical signal to the first input terminal 308 while grounding the first input terminal 307, a transformed electrical signal is applied between the first and second output terminals 309 and 310.
Similar to the transformer element, a differential inductor element, which is another example of the mutual induction circuit, has tended to be incorporated into the integrated circuit. Two conventional differential inductor elements will be described below.
FIG. 35 is a circuit diagram illustrating a differential switch circuit including a differential inductor element as a fourth exemplary conventional mutual induction circuit. FIG. 36 is a circuit diagram of a differential distributed amplifier circuit including a differential inductor element as a fifth exemplary conventional mutual induction circuit. In a simple comparison with a single-phase circuit, a differential circuit, such as the differential switch circuit shown in FIG. 35 or the differential distributed amplifier circuit shown in FIG. 36, requires twice the number of elements. In particular, an inductor element occupies a larger area relative to other types of elements. Accordingly, in the case of the above-mentioned differential circuit with high element density, the inductor element is a factor in increasing various costs. In order to address the above problem, Japanese Patent Laid-Open Publication No. 2002-164704 proposes a differential inductor element as described below.
FIGS. 37A and 37B are perspective views each illustrating the structure of the differential inductor element as the fifth exemplary conventional mutual induction circuit. In FIG. 37A, the differential inductor element includes two spiral inductor elements arranged in a vertical direction. Each spiral inductor element receives and outputs a balanced signal equivalent in amplitude but reversed in phase with respect to that received and outputted by the other spiral inductor element.
More specifically, a first spiral inductor includes a input wiring conductor 604a, a spiral wiring conductor 601a wound in a spiral form, and an output wiring conductor 605a for outputting a signal. Similarly, a second spiral inductor includes an input wiring conductor 604b, a spiral wiring conductor 601b, and an output wiring conductor 605b. In the above first and second spiral inductors, the spiral wiring conductors 601a and 601b are wounded in opposite directions, and are formed in upper and lower layers so as to overlap with each other via an insulating layer.
The input wiring conductor 604a is connected to the spiral wiring conductor 601a via a lead conductor 602a, and the input wiring conductor 604b is connected to the spiral wiring conductor 601b via a lead conductor 602b. The lead conductor 602a is formed in a wiring layer underlying a wiring layer in which the spiral wiring conductor 601a is formed, and the lead conductor 602b is formed in a wiring layer underlying a wiring layer in which the spiral wiring conductor 601b is formed. Interlayer contacts 603a through 603d are used for connections between different wiring layers.
In the differential inductor element of FIG. 37B, the spiral wiring conductors 601a and 601b are wounded in opposite directions, and the spiral wiring conductors 601a and 601b, excluding intersections 606a through 606c, are alternately arranged in the same wiring layer so as to be parallel to each other.
The differential inductor element as shown in FIGS. 37A and 37B is realized in an area approximately equivalent of an area occupied by one inductor element.
In some cases, a high frequency circuit, typified by a radio circuit incorporated into an integrated semiconductor circuit, is realized by a differential circuit in order to reduce common mode noise. However, in a conventional transformer element, coils are not symmetrical to each other when viewed from the signal input side. Accordingly, even if in-phase and reverse-phase signals contained in a differential signal are respectively supplied to two input terminals, there arises a problem that two signals, which are reversed in phase with respect to each other, might not be obtained from the two output terminals.
Note that if the above-described conventional transformer elements (see FIGS. 32A and 32B) are used in even numbers, it is possible to realize the symmetry as described above. However, there arises another problem that the transformer elements occupy a large area of a semiconductor integrated circuit.
In order to reduce internal losses due to resistive components of a semiconductor substrate, the transformer element is generally formed in a wiring layer located as far away from the semiconductor substrate as possible. A conventional transformer element requires three or more wiring layers. For example, in the first mutual induction circuit 100, one wiring layer is required for each of the primary and secondary coils 101 and 102. Moreover, each of the primary and secondary coils 101 and 102 has one terminal at its spiral center side, and therefore an additional wiring layer is required for a signal line for supplying an input signal or outputting an output signal. Similarly, the second transformer element 200 includes the coils 208 and 205, which are shaped similar to the primary and secondary coils 101 and 102, respectively, and therefore requires three winding layers. As for the transformer element 300, three wiring layers are required only for forming three planar spiral coils 302 through 304.
As is apparent from the foregoing, a considerable number of wiring layers are required for forming a conventional transformer element. Moreover, only a limited number of wiring layers can be formed in a semiconductor process. Accordingly, there are difficulties in forming the conventional transformer element sufficiently away from the semiconductor substrate so as to reduce internal losses due to resistive components of the semiconductor substrate.
Similarly, in a conventional differential inductor element, two inductors are not formed in a symmetric manner. Accordingly, even if in-phase and reverse-phase signals contained in a differential signal are respectively supplied to two input terminals, there arises a problem that two signals, which are reversed in phase with respect to each other, might not be obtained from the two output terminals. As in the case of the conventional transformer element, if the conventional differential inductor element is used in even numbers, it is possible to realize the symmetry as described above. However, there arises another problem that the differential inductor elements occupy a large area of a semiconductor integrated circuit.